Traditionally, a light emitting diode (LED) is fabricated into a parallelepiped shape. An LED usually has a small total reflection critical angle because there is a great difference between the refractive indexes of a semiconductor and a packaging material. The light generated by LED reaching an interface by an angle greater than the total reflection critical angle will be totally reflected back to the interior of the LED chip. Besides, the parallel faces of a parallelepiped decrease the probability that light leaves a semiconductor from an interface. Thus, photons can only be totally reflected inside a chip until they are completely absorbed and converted into heat. Therefore, LED usually has insufficient light efficiency.
Changing LED shape is an effective approach to improve LED light efficiency. In a U.S. Pat. No. 6,229,160, HP and LumiLeds Co. disclosed a “Truncated Inverted Pyramid (TIP)” LED, wherein four faces of an AlGaInP/GaP LED chip is mechanically fabricated to be no more parallel to each other. Thus, the external quantum efficiency thereof is greatly increased to 55%, and the light efficiency thereof can reach as high as 1001 m/W. The TIP LED is the first LED achieving the above mentioned standard. However, the technology of TIP LED can only apply to red light AlGaInP/GaP LED chips. A gallium nitride (GaN) LED is epitaxially grown on a sapphire substrate, which is very hard to mechanically fabricate. Therefore, the TIP LED technology cannot apply to GaN LED.
A U.S. Pat. No. 6,768,136 disclosed a LED using a SiC or GaN substrate, which can be fabricated more easily than sapphire. Thus, LED chips can be mechanically fabricated to change their shapes and promote light efficiency. Thereby, the light efficiencies of InGaN LED and GaN LED can be doubled. However, the prices of SiC substrates and GaN substrates are very high. Therefore, this technology is hard to commercialize.
A R.O.C. patent publication No. 565957 disclosed a “Hydride Vapor Phase Epitaxy (HVPE)”, wherein a thick-film GaN epitaxial layer with inclined natural planes is formed on a substrate, and LED crystal is then formed on the substrate with a MOCVD (Metal Organic Chemical Vapor Deposition) technology. Thereby, LED has a higher light efficiency. This technology indeed solves the problems of mechanical fabrication and substrate price. However, it has the disadvantages of needing two epitaxial processes, which make LED fabrication more complicated.